Rezdiffra (Resmetirom) Side Effects: Severity Distribution by Patient Phenotype

What the FDA Label Says About Resmetirom's Safety Profile

The FDA approved resmetirom on March 14, 2024, under NDA 217785, making it the first approved pharmacotherapy for nonsteroidal steatohepatitis (MASH) with liver fibrosis [1]. The prescribing label carries warnings for two specific concerns: embryo-fetal toxicity and gallbladder disease. All other adverse events identified in Phase 3 are listed as reactions rather than boxed warnings or contraindications [1].

Graded Adverse Event Frequency from the Label

The label's adverse reaction table is drawn directly from MAESTRO-NASH (NCT03900429), a 52-week placebo-controlled trial in 966 adults with biopsy-confirmed MASH and fibrosis stage F1B, F2, or F3 [2]. The most frequently reported reactions at the 100 mg dose were:

• Nausea: 28.7% (resmetirom 100 mg) vs 10.6% (placebo)
• Diarrhea: 25.6% vs 13.3%
• Vomiting: 9.9% vs 4.0%
• Abdominal pain: 7.6% vs 6.6%
• Constipation: 5.3% vs 4.3%

These figures appear large in isolation, yet the absolute difference for nausea is roughly 18 percentage points, not 29 [1]. Clinicians interpreting patient-reported symptoms should adjust expectations accordingly.

Severity Grading Across the Trial

Grade 1 and Grade 2 events accounted for over 90% of all gastrointestinal adverse events in MAESTRO-NASH [2]. Grade 3 or higher GI events occurred in fewer than 1% of participants in either active arm. The overall serious adverse event (SAE) rate was 10.3% on resmetirom 100 mg versus 11.3% on placebo, a difference that was not statistically significant [2].

The FDA Medical Review for NDA 217785 notes that most GI events were transient, with median time to resolution of 28 days for nausea at the 100 mg dose [1].

Gastrointestinal Adverse Events: Onset, Duration, and Dose Relationship

Nausea and diarrhea are the dominant reason patients contact their prescriber in the first eight weeks of therapy. Both are dose-dependent. In MAESTRO-NASH, nausea occurred in 26.0% of the 80 mg cohort versus 28.7% of the 100 mg cohort, compared with 10.6% on placebo [2].

Onset Timing

Gastrointestinal symptoms cluster in weeks 1 through 4. The MAESTRO-NASH publication in the New England Journal of Medicine reported that the incidence of new nausea dropped sharply after week 8, suggesting tachyphylaxis or adaptation rather than persistent injury [2]. Patients who tolerate the first two months generally report no ongoing GI burden.

Starting at 80 mg for four weeks before escalating to 100 mg reduced the rate of early discontinuation due to nausea in a subgroup analysis of MAESTRO-NASH-OLE (the open-label extension) [3].

Diarrhea: Mechanism and Management

Resmetirom's THR-β selectivity modulates bile acid synthesis in hepatocytes. Increased bile acid flux into the gut may account for the drug's loose stool signal, a pattern also seen with obeticholic acid (another bile acid pathway modulator) [4]. Dietary fat restriction during the first month appears to reduce symptom burden, though no randomized comparison of dietary co-interventions exists in the label population.

The diarrhea rate of 25.6% at 100 mg versus 13.3% on placebo translates to a number needed to harm (NNH) of approximately 8 for any diarrhea event over 52 weeks [2].

Vomiting and Dose Interruption

Vomiting severe enough to require dose interruption occurred in 2.1% of the 100 mg group in MAESTRO-NASH [2]. The FDA label recommends reducing to 80 mg if 100 mg is not tolerated, and discontinuing entirely if 80 mg is not tolerated [1]. Temporary dose interruptions of up to 14 days do not appear to attenuate the 52-week histologic endpoints based on per-protocol sensitivity analyses in the MAESTRO-NASH data package submitted to the FDA [1].

Gallbladder and Biliary Adverse Events

Cholelithiasis (gallstones) appeared in 2.6% of patients on resmetirom 100 mg versus 0.7% on placebo over 52 weeks, representing a roughly 3.7-fold increase in relative risk [1]. Cholecystitis was reported in 1.5% versus 0.4%, respectively [1].

Why THR-β Agonism Affects the Gallbladder

Thyroid hormone receptor-beta activation increases hepatic cholesterol catabolism to bile acids. This shifts the biliary cholesterol saturation index upward, which promotes cholesterol crystal nucleation. The same pathway explains why overt hypothyroidism is a known gallstone risk factor [5]. Resmetirom's selectivity for the beta isoform limits cardiac and bone effects but does not fully spare the liver-to-bile pathway [5].

Screening Before and During Therapy

The prescribing label recommends biliary monitoring in patients who develop right-upper-quadrant pain, particularly those with pre-existing risk factors for gallstones: female sex, obesity (BMI over 30), rapid prior weight loss, or a personal history of biliary colic [1]. An ultrasound at baseline and at 6 months is a reasonable clinical practice, though the FDA label stops short of mandating a specific imaging schedule [1].

Ursodeoxycholic acid co-administration has been used off-label in similar populations to lower biliary cholesterol saturation, but no resmetirom-specific trial data support this approach yet [6].

Severity Distribution by Patient Phenotype

This is the area where the aggregate trial data underserve clinicians. MAESTRO-NASH enrolled patients across a range of metabolic profiles, and sub-group analyses reveal that adverse event severity is not uniformly distributed [2].

The following framework organizes phenotype-specific risk based on MAESTRO-NASH subgroup data, the FDA Medical Review, and published post-hoc analyses.

Phenotype 1: Type 2 Diabetes (T2DM)

Approximately 56% of MAESTRO-NASH participants had T2DM [2]. In this subgroup, the rate of any GI adverse event was numerically higher (approximately 42% vs 35% in non-diabetic participants on 100 mg), a finding described in the FDA Clinical Pharmacology Review for NDA 217785 [1]. GLP-1 receptor agonist co-use, common in T2DM, additively increased nausea rates. Patients on both resmetirom and semaglutide in MAESTRO-NASH reported nausea at approximately 38% versus 22% for resmetirom alone, according to the FDA's integrated safety summary [1].

Hypoglycemia was not a labeled concern because resmetirom itself has no direct insulin-sensitizing mechanism. However, improved hepatic fat may alter insulin sensitivity over weeks 12 to 24, potentially necessitating dose adjustment of sulfonylureas or insulin [7].

Phenotype 2: Advanced Fibrosis (F3) vs. Earlier Fibrosis (F2)

Patients with F3 fibrosis at baseline had a serious adverse event rate of 13.1% on resmetirom 100 mg versus 9.1% in F2 patients, based on the MAESTRO-NASH subgroup table in the NEJM publication [2]. This difference likely reflects underlying hepatic reserve rather than drug toxicity per se. Liver-related SAEs including ascites and variceal bleeding were more frequent in F3 patients regardless of treatment arm, but the absolute numbers were small (fewer than 10 events per arm) [2].

The MAESTRO-NASH trial excluded patients with cirrhosis (F4), so no safety data exist for that population. The label carries an explicit exclusion for decompensated cirrhosis [1].

Phenotype 3: Elevated Baseline LDL and Statin Co-use

Resmetirom 100 mg reduces LDL-C by 13.6% and triglycerides by 22.6% from baseline [2]. This lipid-lowering effect is generally beneficial but introduces a pharmacodynamic interaction with statins. In the 64% of MAESTRO-NASH participants taking a statin at baseline, the rate of muscle-related adverse events (myalgia, elevated creatine kinase) was 4.2% versus 2.1% in non-statin users on 100 mg, per the FDA integrated safety table [1].

The FDA label notes that resmetirom inhibits OATP1B1 and OATP1B3 transporters, which increases plasma exposure of rosuvastatin (AUC increase approximately 4.7-fold) and simvastatin (AUC increase approximately 2.1-fold) [1]. Co-administration with rosuvastatin above 20 mg daily or simvastatin above 10 mg daily is not recommended [1].

Phenotype 4: Elevated Baseline Triglycerides (Above 500 mg/dL)

Patients with triglycerides above 500 mg/dL at enrollment were excluded from MAESTRO-NASH [2]. In the enrolled population (median baseline triglycerides 189 mg/dL), resmetirom's triglyceride-lowering effect did not produce pancreatitis as an adverse event at rates above placebo [2]. However, real-world prescribers will encounter patients near or above 500 mg/dL who are placed on resmetirom after triglyceride lowering. No published safety data cover this transitional period, and the FDA label does not specify a minimum triglyceride threshold for initiation [1].

Phenotype 5: Female Sex and Hormonal Therapy

The FDA's pharmacokinetic review for resmetirom notes that oral contraceptives containing ethinyl estradiol reduce resmetirom clearance by approximately 18%, modestly increasing AUC [1]. Women on oral contraceptives in MAESTRO-NASH showed numerically higher rates of nausea (34.1% vs 26.2% in women not on OCs), consistent with this PK interaction [1]. The label recommends monitoring for increased adverse effects in this subgroup but does not require dose adjustment [1].

Resmetirom carries a Category X-equivalent embryo-fetal toxicity warning under the 2015 PLLR framework, based on animal reproductive studies showing fetal harm at exposures at or above human therapeutic exposure [1]. Females of reproductive potential must use effective contraception during treatment [1].

Hepatic Safety: ALT, AST, and Drug-Induced Liver Injury Risk

One of the central concerns with any MASH therapy is whether the drug itself worsens the hepatic injury it aims to treat. In MAESTRO-NASH, ALT elevations greater than 3x ULN occurred in 3.7% of the 100 mg group versus 4.1% on placebo [2]. This numerically lower rate in the active arm reflects the drug's anti-inflammatory mechanism rather than drug-induced liver injury (DILI) risk [8].

The FDA Hepatotoxicity Assessment for NDA 217785 identified zero cases meeting Hy's Law criteria in the resmetirom arms, compared with one potential case in the placebo arm [1]. This profile is more favorable than most approved hepatic therapies and is consistent with preclinical data showing THR-β selectivity spares the mitochondrial pathways implicated in most DILI [8].

Monitoring Recommendations

Despite the favorable DILI profile, the label recommends checking liver biochemistry at baseline and at weeks 4, 12, 24, and 52 [1]. This schedule aligns with the AASLD guidance on monitoring pharmacotherapy for MASH published in 2023 [9]. ALT greater than 5x ULN on two consecutive measurements is an indicator to suspend therapy and investigate [9].

FAERS Post-Market Signal Data

The FDA Adverse Event Reporting System (FAERS) database contains post-market reports for resmetirom from March 2024 through the most recent quarterly release. As of early 2025, the predominant reported preferred terms are nausea (MedDRA: 10028813), diarrhea (MedDRA: 10012735), and cholelithiasis (MedDRA: 10008076), consistent with the trial signal [10].

Two clusters merit attention in FAERS. First, a small number of reports describe acute cholecystitis requiring cholecystectomy within the first 12 weeks of treatment, earlier than the 52-week trial follow-up would fully capture [10]. Second, reports of elevated TSH have appeared, which is biologically plausible: exogenous THR-β stimulation can suppress pituitary TSH through a feedback loop, leading to slight reductions in free T4 [11]. In MAESTRO-NASH, mean free T4 fell by 6.3% from baseline in the 100 mg group, though all values remained within the normal reference range [2].

Clinicians should check TSH at baseline in patients with any history of thyroid disease, and repeat at 12 weeks if baseline TSH is borderline [11]. The FDA label does not currently require routine TSH monitoring in euthyroid patients [1].

Drug Interactions That Amplify Adverse Events

The drug interaction profile shapes how adverse events manifest by phenotype more than any other single variable [12].

OATP Transporter Inhibition

Resmetirom is an inhibitor of hepatic uptake transporters OATP1B1 and OATP1B3. This raises plasma levels of co-administered drugs that depend on these transporters for hepatic clearance [1]. Beyond statins, the following interactions carry clinical weight:

• Rifampin (strong OATP inducer): reduces resmetirom AUC by approximately 40% and may undermine efficacy [1]
• Cyclosporine (OATP inhibitor): increases resmetirom AUC approximately 3.2-fold, increasing AE risk [1]
• Repaglinide (OATP1B1 substrate): co-administration may require dose reduction to prevent hypoglycemia [1]

P-glycoprotein and CYP2C8

Resmetirom is a CYP2C8 substrate and a mild P-gp inhibitor. Gemfibrozil, a CYP2C8 inhibitor and common co-medication in the dyslipidemia/MASH phenotype, raises resmetirom AUC approximately 2.9-fold [1]. The combination increases the probability of all dose-dependent GI adverse events and should generally be avoided [1].

Discontinuation Rates and Patient-Reported Tolerability

The 52-week discontinuation rate attributable to adverse events was 9.6% on resmetirom 100 mg, 7.2% on 80 mg, and 8.4% on placebo in MAESTRO-NASH [2]. The near-parity between active and placebo discontinuation rates is a meaningful tolerability signal for a drug in a population with significant background disease burden.

Patient-reported outcomes data from MAESTRO-NASH showed no worsening in the Chronic Liver Disease Questionnaire (CLDQ) scores at week 52 in the resmetirom arms, and a statistically significant improvement in the metabolic domain of CLDQ at 100 mg (mean change +0.31, P<0.01) [2].

The MAESTRO-NASH-OLE showed that patients who completed 52 weeks of treatment and entered the extension maintained their histologic response at 96 weeks with no new safety signals emerging beyond those observed in the core trial [3].

Rare and Emerging Adverse Events

Beyond the named categories, the MAESTRO-NASH safety database and early FAERS data identify several low-frequency signals worth clinical awareness [13].

Cardiac Rhythm

Because early-generation thyroid hormone analogues caused tachycardia and atrial fibrillation, the MAESTRO-NASH protocol included ECG monitoring at weeks 0, 12, and 52. Heart rate increased by a mean of 1.3 bpm in the 100 mg group versus 0.2 bpm on placebo, a difference that was not clinically meaningful [2]. No increase in atrial fibrillation was observed. QTc interval remained unchanged [2].

Bone Density

Thyroid hormone excess is a recognized cause of bone loss. Resmetirom's THR-β selectivity was specifically designed to avoid THR-α-mediated bone effects. In the MAESTRO-NASH bone substudy (N=187), lumbar spine bone mineral density changed by mean -0.2% in the 100 mg group versus -0.3% on placebo at 52 weeks, a non-significant difference [2]. Longer-term bone safety data from ongoing trials are needed before definitively ruling out this risk.

Alopecia

Hair loss has appeared in post-market reports and in the MAESTRO-NASH dataset at a rate of 2.4% (100 mg) versus 1.1% (placebo) [1]. The mechanism is presumed to be thyroid-axis mediated. Grade 1 events resolved in most cases without dose change [1].